Managed Debugging Assistant 'PInvokeStackImbalance'

Question

I have a C function I'm exposing to F# code:

#define EXPORT extern "C" __declspec(dllexport)
EXPORT Video *NewVideoEncoder(LPCWSTR file, unsigned int width, unsigned int height)

The client F# code is:

module VideoFunctions =
    open System
    open System.Runtime.InteropServices

    [<DllImport(Common.LibraryName)>]
    extern IntPtr NewVideoEncoder(IntPtr filename, uint32 width, uint32 height)

let strPtr = Marshal.StringToHGlobalUni(filename)
let video = VideoFunctions.NewVideoEncoder(strPtr, uint32 width, uint32 height)

When I can the code I get a "Managed Debugging Assistant 'PInvokeStackImbalance'" warning. The code appears to execute correctly, but I observe some weird behaviour with later PInvoke calls.

The docs say the most likely cause of this is a signature mismatch, but to me two signatures look identical.